There is little doubt that we are in the midst of a worldwide epidemic of diabetes. Almost 16 million people in the US are thought to be afflicted, a third of whom are undiagnosed. Insulin resistance is recognized as a defined characteristic trait of the disease, by the inability to respond to normal circulating levels of insulin. The primary lesion in this state involves defects in the uptake and storage of glucose in muscle and fat cells. Targeting these defects holds the key to the development of new therapeutic approaches. However, understanding the specific lesions that cause insulin resistance in patients with type 2 diabetes will first require a better grasp of the cell biology of insulin action. To this end, the molecular events involved in the regulation of glucose uptake by insulin will be investigated, with special attention to the underlying basis for the specificity of actions of the hormone. Recent data has revealed the activation of a novel tyrosine phosphorylation pathway that is localized to discrete domains of the plasma membrane called lipid rafts. Aim 1 will evaluate the role of the primary insulin receptor substrate in this pathway, the protooncogene c-cbl. Mutations in cbl and its binding partners crk and C3G will be introduced to determine whether this pathway plays an essential role in insulin action. In Aim 2 the adapter protein CAP that enables the receptor to phosphorylate cbl will be studied. Analysis of the structure/function relationship of CAP will depend on the development of deletion and chimeric mutants, in order to further evaluate the importance of the pathway. Aim 3 will focus on the new concept that there are two pools of insulin receptor that are spatially segregated in the plasma membrane, each linked to different signaling pathways. Finally, the physiological role of the pathway will be studied in Aim 4 by the targeted disruption of the CAP gene in mice in a tissue-specific manner. Together, these approaches will allow for the evaluation of the importance of this novel pathway in insulin action, setting the stage for future investigations into its potential role in the development of diabetes.